Photodiodes are widely used in many modern day electronic devices to convert light into an electric signal (e.g., a voltage or a current). A photodiode is a p-n junction (i.e., an interface between a p-type region and an n-type region) or PIN structure within a semiconductor substrate. Photodiodes are typically formed within an exposed surface of a semiconductor substrate to allow the photodiode to receive light.
During operation, when a photon of sufficient energy strikes a photodiode it excites an electron within a semiconductor substrate of the photodiode, thereby generating an electron-hole pair. As electrons and holes are generated, the holes move in the semiconductor substrate in an opposite direction as the electrons to generate a current.